In recent years, as semiconductor devices have enhanced their operations and performance, semiconductor packages have become miniaturized. To achieve this miniaturization, so-called BGA (Ball Grid Array) packages, in which solder balls serving as external connection terminals are attached on the bottom surface of a semiconductor package, are being developed. In the case of this BGA package, a semiconductor chip is mounted on the principal surface of an insulated substrate, and solder balls are attached to the rear surface. Conductor patterns to be connected to the semiconductor chip via wires are formed on the principal surface of the insulated substrate, and the solder balls are connected to said conductor patterns through via holes formed on the insulated substrate.